Add a note about changelogs

Summary:
LukeDefeo called out that there's currently no guide for how to create changelog items.

I couldn't find another good page to add this to. Might be a good idea to expand this with further tips for making good contributions.

Reviewed By: LukeDefeo, nikoant

Differential Revision: D36702634

fbshipit-source-id: 15bc1dcf420ea923a714929eff8ac817efa56ceb

website/sidebars.js

@@ -140,6 +140,7 @@ module.exports = {
   internals: {
     Internals: [
       'internals/index',
+      'internals/contributing',
       'extending/public-releases',
       'extending/testing-rn',
       'internals/linters',